Breathing apparatuses have breathing masks to which different air sources can be attached: filter, power-assisted filtering device, compressed air breathing apparatus. The combination of breathing mask and air source yields breathing apparatuses which are referred to as follows in U.S. American parlance:                air purifying respirator (APR): breathing mask+filter        powered air purifying respirator (PAPR): breathing mask+power-assisted filtering device        self-contained breathing apparatus (SCBA): breathing mask+compressed air breathing apparatus.        
These breathing apparatuses differ for the user primarily by the protection factor that is provided. The breathing masks are tailored to the apparatus that it is to be attached to, a mask for filter use is accordingly not the same in terms of construction as a mask for use for compressed air breathing. There are apparatuses that are self-contained: that is to say, it is generally not possible to attach a filter to a compressed air breathing mask. Because a compressed air breathing apparatus exists as a normal pressure and an overpressure (positive pressure) variant, a further differentiation between mask types is necessary in this case: normal pressure masks and overpressure masks. In the case of the latter, the exhalation valve is biased with a spring force in order to achieve a static overpressure in the mask. This increases the protection factor, because a positive pressure gradient is thus maintained from the inside to the outside.
Some use scenarios require the use of different types of breathing apparatus. So-called hybrid masks offer the possibility of selectably using one of two of the breathing apparatuses mentioned at the beginning in one breathing apparatus. The combinations APR/SCBA or PAPR/SCBA are common. The compressed air breathing apparatus is thereby generally in the form of an overpressure system, the other two apparatus types are, by definition, normal pressure systems. For the breathing mask, it is thereby necessary to be able to change the exhalation valve from a normal pressure state into an overpressure state and vice versa. In the case of such hybrid apparatuses, it is necessary to disconnect one air source (e.g. compressed air breathing apparatus) and connect another air source (e.g. filter—filtered ambient air).
U.S. 2007/0251525 A1 discloses a known breathing apparatus which can be switched from an operating state in which ambient air drawn in through a filter serves as the air source, to an operating state in which the breathing apparatus is supplied by a compressed air breathing apparatus, and vice versa. The operating state of the breathing apparatus is determined by the switch position of the compressed air cylinder valve (open or closed). The known breathing apparatus has a compressed air source (e.g. compressed air cylinder) with a compressed air source valve. This compressed air source valve is referred to as an actuator and has two functions in the known breathing apparatus:    1. opening the valve leads to the flow of compressed air and thus places the pneumatic system of the breathing apparatus under pressure, closing the valve closes the compressed air source and thus prevents the flow of compressed air;    2. in addition to the mentioned pressure rise in the pneumatic system of the breathing apparatus, opening the valve leads to activation of a pressure adjusting mechanism and thus to switching of the exhalation valve from the first operating state into the second operating state.
According to the description of the known breathing apparatus, there is a communicative link between the compressed air source valve and the exhalation valve. It is mentioned that information about the opening or closing of the valve can be transmitted in different ways, in particular pneumatically, electrically, hydraulically, mechanically, etc., via the communicative link. If the communication path performs pneumatic transmission of information, opening of the compressed air cylinder valve on the one hand causes the pneumatic system of the breathing apparatus to be placed under pressure and on the other hand causes the exhalation valve to be moved into the second operating state, in which a higher internal pressure is built up in the breathing mask.
Such a configuration of a breathing apparatus has the advantage that, by operation of a single actuating member (compressed air cylinder valve), both the compressed air cylinder valve is opened and the exhalation valve is moved into the second operating state, so that a higher internal pressure is built up in the breathing mask. On the other hand, this has the disadvantage that the pneumatic system of the breathing apparatus is not placed under pressure until switching to compressed air breathing is actually to be carried out. This is then effected only in a situation in which the wearer of the breathing apparatus is moving in an increasingly more hazardous environment, when the wearer wishes to change from breathing via the filter to compressed air breathing. This has the disadvantage that a fault in the build-up of pressure in the pneumatic system of the breathing apparatus caused by any type of malfunction downstream of the compressed air source valve does not manifest itself as incomplete or failed build-up of pressure in the pneumatic system of the breathing apparatus until the time when compressed air breathing is actually required.